<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .test>div{
      float: left;
      height: 500px;
    }
    .test>div:nth-child(1){
      background-color: red;
    }
    .test>div:nth-child(2){
      background-color: green;
    }
    .test>div:nth-child(3){
      background-color: blue;
    }
    @media screen and (min-width: 200px) and (max-width: 400px) {
      .test>div{
        width: 100%;
      }
    }
    @media screen and (min-width: 401px) and (max-width: 800px) {
      .test>div{
        width: 50%;
      }
    }
    @media screen and (min-width: 801px) {
      .test>div{
        width: 33.3%;
      }
    }

  </style>
</head>
<body>
  <div class="test">
    <div></div>
    <div></div>
    <div></div>
  </div>
</body>
</html>
